A large motor vehicle, such as a heavy truck, has substantial kinetic energy when in motion. When the accelerator pedal that has been depressed to accelerate the vehicle is released, the powertrain itself becomes a load that decelerates the vehicle. While foundation brakes at the wheels are the primary means for stopping the vehicle, one or more deceleration devices and/or systems that when operated can impose a load on the driven wheels may also be available for decelerating the vehicle. Examples of such devices include exhaust brakes, engine brakes, and driveline retarders. Downshifting of the transmission increases the braking load that the engine imposes on the drivetrain, and can also be used to decelerate the vehicle.
When actuated, the foundation brakes apply friction forces to rotating wheels, creating torque that opposes wheel rotation. That torque creates forces at the interfaces between rubber tires on the wheels and the underlying road surface that oppose the direction of vehicle motion along the road, thereby decelerating the vehicle. Waste heat is generated both at the foundation brakes and at the tire/road interfaces.
A strategy and method that can recapture and store energy from what would otherwise become waste heat for later use to accelerate a vehicle is obviously desirable for a number of reasons, one of which is improved fuel economy and another of which is reduction in wear on the foundation brakes. Even an ability to recapture what might be seen by some as fairly small amounts of energy can be meaningful to the operation of fleets of commercial vehicles like large trucks and highway tractors.
Certain motor vehicles (commonly called hybrids) have energy storage means, such as pressurized fluid vessels or storage batteries that store potential energy that was converted from kinetic energy as the vehicle decelerated (such method of deceleration sometimes being called regenerative braking). A vehicle that is equipped with two different types of energy recapture systems is sometimes called a dual-hybrid vehicle. If those systems are capable of recapturing sufficient energy to satisfy a deceleration request, foundation brakes need not be applied.
A strategy and method that coordinates the operation of certain engine/powertrain deceleration systems with the operation of foundation brakes for recapturing what would otherwise become waste energy should give precedence to regenerative braking when vehicle operating conditions make it appropriate to do so. If a braking request cannot be satisfied by regenerative braking alone, or when the on-board energy storage means cannot accept any more energy for storage, then the use of engine/powertrain deceleration systems that do not store, but rather simply dissipate, energy may become appropriate before foundation brakes are applied. In other words, when a vehicle has various on-board systems and/or devices that are potentially available for decelerating a moving vehicle, including energy recapture and storage systems, a preferential order for using such devices to satisfy a braking request may be: use of the energy recapture and storage systems; use of energy-dissipating systems and/or devices other than foundation brakes; and lastly, foundation brakes themselves.
One system and method for decelerating a moving vehicle by recapture of kinetic energy comprises coupling an electric motor/generator through a portion of the vehicle drivetrain to driven wheels and controlling the degree of coupling in a manner that causes kinetic energy of the moving vehicle to be used to operate the motor/generator as a generator delivering electric current to recharge an on-board bank of storage batteries. Depending on conditions under which the vehicle is being driven and/or state of battery bank charge, the battery bank may be unable to accept all the kinetic energy that needs to be dissipated in order to satisfy a particular braking request, therefore requiring that energy-dissipating systems or devices such as foundation brakes be used to make up the difference.
Another system and method for decelerating a moving vehicle by recapture and storage of energy comprises coupling a hydraulic pump/motor (hydraulic motor/generator) through a portion of the drivetrain to driven wheels and controlling the degree of coupling in a manner that causes kinetic energy of the moving vehicle to operate the hydraulic motor/generator as a pump to charge a hydraulic accumulator. Depending on conditions under which the vehicle is being driven and/or the accumulator state-of-charge, the accumulator may be unable to accept all the kinetic energy that needs to be dissipated in order to satisfy a particular braking request, also requiring that energy-dissipating systems or devices such as foundation brakes be used to make up the difference.